The present invention relates to a device for purifying exhaust gas of diesel engines and, more particularly, to a device for purifying exhaust gas by removing particulate matters and nitrogen oxides from the exhaust gas.
Vehicles mounting diesel engines have a serious problem of reducing particulate matters (hereinafter referred to as PMs) and nitrogen oxides (hereinafter referred to as NOx) contained in the exhaust gas emitted from the diesel engines. In order to decrease PMs and NOx, attempts have been made to remove PMs and NOx by an after-treatment system in addition to suppressing the generation of PMs and NOx by improving the combustion by means of a high-pressure injection and exhaust gas recirculation system (hereinafter referred to as EGR).
As a device for removing PMs, there has been known a diesel particulate filter (hereinafter referred to as DPF). The DPF is constituted by a porous honeycomb or a felted ceramic fiber, and traps PMs contained in the exhaust gas. The trapped PMs are continuously burned with NO2 formed by oxidizing the NO in the exhaust gas through an oxidizing catalyst disposed on the upstream of the DPF. The so-called continuous regeneration type DPF which continuously regenerates the DPF by utilizing the oxidizing catalyst permits the PM to burn at a temperature which is as low as 400xc2x0 C. or lower. Hence, it offers an advantage in that the entire apparatus can be simply and compactly fabricated as compared to the regeneration of the DPF by heating using an electric heater or the like.
Further, an Nox-reducing catalyst has been known as a device for removing NOx. In order to reduce the NOx by using the NOx-reducing catalyst, however, the exhaust gas must be rendered to be a reducing atmosphere at all times. In the case of a diesel engine which emits the exhaust gas creating an oxygen-rich atmosphere, it is necessary to continuously inject the fuel that does not contribute to increasing the engine output, by executing a so-called post injection (injection in a very small amount in the latter half of the combustion stroke), so that the exhaust gas is rendered to be a reducing atmosphere. This, however, causes a problem of making fuel consumption worse.
Under the circumstances, a study has been made on a so-called NOx occluding/reducing catalyst which occludes NOx when the exhaust gas has a lean air-fuel ratio and releases the occluded NOx to reduce it into N2 when the exhaust gas has a rich air-fuel ratio. The NOx occluding/reducing catalyst has a constitution that an Nox occluding substance and a catalytically active component which are carried on a carrier, and it oxidizes the NOx in the exhaust gas on the catalytically active component so as to allow the NOx occluding substance to occlude NOx when the exhaust gas has a lean air-fuel ratio, i.e., when the exhaust gas is an oxidizing atmosphere, and releases the NOx occluded by the NOx occluding substance to reduce the Nox to N2 on the catalytically active component by using HC and CO in the exhaust gas as reducing agents when the exhaust gas has a rich air-fuel ratio, i.e., when the exhaust gas is a reducing atmosphere. By using the NOx occluding/reducing catalyst as described above, the exhaust gas may be rendered to be a reducing atmosphere only when the NOx occluded by the NOx occluding substance is released to be reduced, giving advantage of superior fuel consumption property to that of the conventional NOx-reducing catalyst.
By furnishing a diesel engine with the above-mentioned continuous regeneration type DPF utilizing the oxidizing catalyst and with the NOx occluding/reducing catalyst, it is allowed to remove PMs and NOx from the exhaust gas simultaneously. Devices for purifying exhaust gas of diesel engines equipped with such devices have been disclosed in, for example, Japanese Laid-open Patent Publication (Kokai) Nos. 53442/1997 (JP-A 9-53442) and 137716/1997 (JP-A 9-137716).
By the way, when the diesel engine is furnished with the DPF of the continuous regeneration type utilizing the oxidizing catalyst and with the NOx occluding/reducing catalyst, it is necessary that these devices are arranged in order of the oxidizing catalyst, DPF and NOx occluding/reducing catalyst from the upstream side of the exhaust gas passage, since NO is formed at the time when the PMs burn on the DPF. Therefore, the exhaust gas passes through the oxidizing catalyst prior to passing through the NOx occluding/reducing catalyst. Here, even when the exhaust gas having a rich air-fuel ratio passes through the oxidizing catalyst, the HC and CO in the exhaust gas are oxidized to CO2 and H2O due to the presence of oxygen in the exhaust gas though its amount may be small. Therefore, when the oxidizing catalyst is disposed on the upstream side of the NOx occluding/reducing catalyst, the HC and CO which are the reducing agents are not supplied in sufficient amounts to the NOx occluding/reducing catalyst when the NOx is to be reduced through the NOx occluding/reducing catalyst by enriching the air-fuel ratio of the exhaust gas. Accordingly, the NOx released from the NOx occluding substance is not reduced but is directly emitted into the atmosphere.
It is an object of the present invention to provide a device for purifying exhaust gas of diesel engines, which is capable of reliably removing the PMs and NOx by furnishing the diesel engines with a DPF of the continuous regeneration type utilizing an oxidizing catalyst and with an NOx occluding/reducing catalyst.
In order to accomplish the above-mentioned object according to the present invention, there is provided a device for purifying exhaust gas of diesel engines, in which an oxidizing catalyst, a diesel particulate filter for trapping particulate matters contained in the exhaust gas, and an NOx occluding/reducing catalyst comprising an NOx occluding substance and a catalytically active component, are successively arranged in an exhaust gas passage of the diesel engine from the upstream side of the passage, said device for purifying exhaust gas of diesel engines comprising:
a bypass that connects the upstream side of said oxidizing catalyst with the downstream side thereof in said exhaust gas passage to bypass said oxidizing catalyst;
an exhaust gas passage change-over means for changing-over the exhaust gas flow to either said oxidizing catalyst or said bypass;
an air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas;
an NOx occlusion amount detector means for detecting the amount of NOx occluded by said NOx occluding substance; and
a controller for controlling said air-fuel ratio control means and said exhaust gas passage change-over means based on a detection signal from said NOx occlusion amount detector means;
wherein said controller controls said air-fuel ratio control means so that the exhaust gas will have a lean air-fuel ratio and controls said exhaust gas passage change-over means so that the exhaust gas will flow through said oxidizing catalyst when the amount of NOx occluded by said NOx occluding substance does not reach a predetermined value, and controls said air-fuel ratio control means so that the exhaust gas will have a rich air-fuel ratio and controls said exhaust gas passage change-over means so that the exhaust gas will flow through said bypass when the amount of NOx occluded by said NOx occluding substance exceeds the predetermined value.